[c-nsp] BGP/ISIS/administrative distance conundrum.

[Jason Lixfeld]

Ah, this worked a treat:

!
ip access-list standard DEFAULT
 permit 0.0.0.0
!
router isis 21949-MANAGEMENT
 vrf management
 net 00.0102.1904.9069.0000.00
 is-type level-2-only
 metric-style wide
 spf-interval 5 1 50
 prc-interval 5 1 50
 lsp-gen-interval 5 1 50
 log-adjacency-changes
 nsf cisco
 default-information originate
 distance 255 0.0.0.0 255.255.255.255 DEFAULT
!

lab#show ip route vrf management

Routing Table: management
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
       + - replicated route, % - next hop override

Gateway of last resort is 72.15.48.10 to network 0.0.0.0

B*    0.0.0.0/0 [200/2] via 72.15.48.10, 02:09:11
      10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
C        10.219.34.0/25 is directly connected, Vlan4009
L        10.219.34.126/32 is directly connected, Vlan4009
i L2     10.219.34.252/32 [115/10] via 10.219.34.124, 02:08:48, Vlan4009
i L2     10.219.34.253/32 [115/10] via 10.219.34.125, 02:08:48, Vlan4009
B        10.219.49.1/32 [200/1] via 72.15.48.10, 02:04:07
B        10.219.49.71/32 [200/1] via 72.15.50.98, 02:11:26
lab#

The ISIS default has been replaced with the BGP default as desired.

Thanks a lot!

On 2012-09-12, at 3:27 PM, Janez Novak <jnovak123 at gmail.com> wrote:

> Hi Jason,
> 
> You can change AD for default route only.
> 
> ip access-list standard ACL-DEFAULT-ROUTE
> permit 0.0.0.0
> 
> router isis
> distance 240 <R1> 0.0.0.0 ACL-DEFAULT-ROUTE
> distance 240 <R2> 0.0.0.0 ACL-DEFAULT-ROUTE
> 
> Kind regards,
> Bostjan
> 
> On Wed, Sep 12, 2012 at 7:02 PM, Jason Lixfeld <jason at lixfeld.ca> wrote:
>> I've been trying to sort out a routing paradox in my lab and I'm hoping someone might have some insight.
>> 
>> Routers 1-4 are all speaking ISIS-L2.
>> Routers 1,2 originate a default route into ISIS.
>> Routers 1,2 speak MP-iBGP northbound and receive a default route from their respective northbound MP-iBGP sessions.
>> Routers 1,2 redistribute ISIS-L2 routes into MP-iBGP so the north side has reachability.
>> 
>> Problem - Because the AD of ISIS is lower than iBGP, R1 and R2 see the ISIS default route an ignore the BGP default route.  This causes a routing loop.
>> 
>> From what I've read, it's not possible to apply some sort of filter to the the ISIS process on R1 or R2 to ignore a learned ISIS default route and just originate a default route.
>> 
>> I can't see any way of adding a route-map to the MP-iBGP sessions on R1 or R2 or either of their northbound MP-iBGP routers to change the AD of the default to less than 115.
>> 
>> I can modify the ISIS distance on R1 and R2 to be greater than iBGP, but that would ultimately lead to all ISIS routes on R1/R2 being ignored in favour of the MP-BGP routes for the same destinations being learned from northbound iBGP.  Likely more routing loops would ensue here.
>> 
>> I could somehow tag the ISIS routes that are redistributed into MP-BGP on R1/R2 and write a route-map to filter those routes from being announced back down to R1 and R2 alleviating the loop.  I'm wondering though, is there's a more straightforward answer that I'm not seeing.
>> 
>> Thanks in advance.
>> 
>> 
>> 
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